Editing Heat transfer (section)

{{Short description|Transport of thermal energy in physical systems}}
{{Use dmy dates|date=August 2020}}
[[File:Convection-snapshot.png|thumb|400px|Simulation of thermal convection in the [[Mantle (geology)|Earth's mantle]]. Colors span from red and green to blue with decreasing temperatures. A hot, less-dense lower boundary layer sends plumes of hot material upwards, and cold material from the top moves downwards.]]

'''Heat transfer''' is a discipline of [[thermal engineering]] that concerns the generation, use, conversion, and exchange of [[thermal energy]] ([[heat]]) between physical systems. Heat transfer is classified into various mechanisms, such as [[thermal conduction]], [[Convection (heat transfer)|thermal convection]], [[thermal radiation]], and transfer of energy by [[phase changes]]. Engineers also consider the transfer of mass of differing chemical species (mass transfer in the form of [[advection]]), either cold or hot, to achieve heat transfer. While these mechanisms have distinct characteristics, they often occur simultaneously in the same system.

Heat conduction, also called diffusion, is the direct microscopic exchanges of kinetic energy of particles (such as molecules) or quasiparticles (such as lattice waves) through the boundary between two systems. When an object is at a different [[temperature]] from another body or its surroundings, [[heat]] flows so that the body and the surroundings reach the same temperature, at which point they are in [[thermal equilibrium]]. Such spontaneous heat transfer always occurs from a region of high temperature to another region of lower temperature, as described in the [[second law of thermodynamics]].

Heat convection occurs when the bulk flow of a fluid (gas or liquid) carries its heat through the fluid. All convective processes also move heat partly by diffusion, as well. The flow of fluid may be forced by external processes, or sometimes (in gravitational fields) by buoyancy forces caused when thermal energy expands the fluid (for example in a fire plume), thus influencing its own transfer. The latter process is often called "natural convection". The former process is often called "forced convection." In this case, the fluid is forced to flow by use of a pump, fan, or other mechanical means.

Thermal radiation occurs through a [[vacuum]] or any [[transparency (optics)|transparent]] [[Optical medium|medium]] ([[solid]] or [[fluid]] or [[gas]]). It is the transfer of energy by means of [[photons]] or [[electromagnetic waves]] governed by the same laws.<ref name=Geankoplis>{{cite book |last=Geankoplis |first=Christie John |year=2003 |title=Transport Processes and Separation Principles |publisher=Prentice Hall |isbn=0-13-101367-X |edition=4th}}</ref>